From 387c513d124694e44057446006b524e4ed367f35 Mon Sep 17 00:00:00 2001 From: Michael Peyton Jones Date: Fri, 15 Mar 2019 17:15:20 +0000 Subject: [PATCH] generic Haskell builder: don't copy packages from GHC In order to build the package databases that we will use when compiling a Haskell package, we iterate over the relevant dependencies, and if they contain a package db, we copy its contents over. So far so good, except when one of those dependencies is GHC. This doesn't happen ordinarily, but it will happen when we construct the package database for compiling `Setup.hs`. This is compiled for the build architecture, so we get the build deps, including both the native and the cross GHC (if there is one). In this case, we end up copying the packages from the GHC's package database. This is at best unnecessary, since we will get those packages from the GHC when we compile with it. At worst, however, this is semantically questionable. We can end up having multiple copies of e.g. Cabal with the same version, but (potentially) different contents. At the moment, GHC will expose one of these at semi-random depending on which one it looks at "first". However, there is a MR open [in GHC](https://gitlab.haskell.org/ghc/ghc/merge_requests/545) which as a side effect will instead expose both, leading to ambiguous module warnings (which is not unreasonable, since it *is* ambiguous). So what can we do about it? The simplest solution is just to not copy the package databases from GHC. GHC is special in this regard, so I think it's okay to treat it specially. This PR should have no effect on anything now, but will prevent any breakage when/if the GHC patch lands. Closes https://github.com/NixOS/nixpkgs/pull/57706. --- pkgs/development/haskell-modules/generic-builder.nix | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/pkgs/development/haskell-modules/generic-builder.nix b/pkgs/development/haskell-modules/generic-builder.nix index e8195668c41c..a6fd7bb0ba96 100644 --- a/pkgs/development/haskell-modules/generic-builder.nix +++ b/pkgs/development/haskell-modules/generic-builder.nix @@ -212,7 +212,16 @@ let nativeGhcCommand = "${nativeGhc.targetPrefix}ghc"; buildPkgDb = ghcName: packageConfDir: '' - if [ -d "$p/lib/${ghcName}/package.conf.d" ]; then + # If this dependency has a package database, then copy the contents of it, + # unless it is one of our GHCs. These can appear in our dependencies when + # we are doing native builds, and they have package databases in them, but + # we do not want to copy them over. + # + # We don't need to, since those packages will be provided by the GHC when + # we compile with it, and doing so can result in having multiple copies of + # e.g. Cabal in the database with the same name and version, which is + # ambiguous. + if [ -d "$p/lib/${ghcName}/package.conf.d" ] && [ "$p" != "${ghc}" ] && [ "$p" != "${nativeGhc}" ]; then cp -f "$p/lib/${ghcName}/package.conf.d/"*.conf ${packageConfDir}/ continue fi